1. Field of the Invention
The present invention relates to a semiconductor thin film having a region substantially regarded as a single crystal (hereinafter called xe2x80x9cmonodomain regionxe2x80x9d) which is formed on a substrate having an insulating surface, and to a semiconductor device using such a semiconductor thin film as an active layer. In particular, the invention relates to a thin-film transistor which uses a crystalline silicon film as an active layer.
2. Description of the Related Art
In recent years, techniques of forming thin-film transistors (TFTs) by using a silicon semiconductor thin film (thickness: hundreds to thousands of angstrom) formed on a substrate having an insulating surface attracted much attention. The thin-film transistor is widely applied to various electronic devices such as ICs and liquid crystal display devices.
The most important portions, i.e., the heart, of the thin-film transistor are the channel-forming region and the junction portions between the channel-forming region and the source and drain regions. That is, it can be said that the active layer most influences the performance of the thin-film transistor.
An amorphous silicon film formed by plasma CVD or low-pressure thermal CVD is commonly used as a semiconductor thin film for constituting the active layer of a thin-film transistor.
At present, thin-film transistors using an amorphous silicon film are in practical use. However, when higher speed operation is required, a thin-film transistor using a silicon thin film having crystallinity (called a crystalline silicon film) is needed.
Examples of known techniques for forming a crystalline silicon film on a substrate are those described in Japanese Unexamined Patent Publication Nos. Hei. 6-232059 and Hei. 6-244103, which were filed by the present assignee. In the techniques described in these publications, a crystalline silicon film that is superior in crystallinity is formed by a heat treatment of 550xc2x0 C. and about 4 hours by utilizing a metal element for accelerating crystallization of silicon.
Further, Japanese Unexamined Patent Publication No. Hei. 7-321339 discloses a technique of causing crystal growth approximately parallel with a substrate by utilizing the above-mentioned techniques. The present inventors call this type of crystallized region a lateral growth region.
A lateral growth region formed by the above technique is a collection of columnar or needle-like crystals that are arranged in the same direction, and hence is superior in crystallinity. It is known that a thin-film transistor whose active layer is formed by using this type of region exhibits high performance.
However, the above technique is still insufficient for formation of thin-film transistors to constitute various arithmetic circuits, memory circuits, etc. This is because the crystallinity is still not sufficiently high to provide the necessary characteristics.
For example, peripheral circuits of an active matrix liquid crystal display device or a passive liquid crystal display device include driver circuits for driving pixel TFTs in the pixel area, a circuit handling or controlling a video signal, a storage circuit for storing various types of information, and other circuits.
Among those circuits, the circuit for handling or controlling a video signal and the storage circuit for storing various types of information are required to have performance equivalent to that of an integrated circuit formed on a known single crystal wafer. Therefore, to integrate the above circuits by using a thin-film semiconductor formed on a substrate, it is necessary to form on a substrate a crystalline silicon film whose crystallinity is equivalent to that of a single crystal.
An object of the invention is to form, on a substrate having an insulating surface, a monodomain region whose crystallinity is equivalent to that of a single crystal. A further object of the invention is to provide a semiconductor device whose active layer is constituted by such a monodomain region.
According to one aspect of the invention, there is provided a semiconductor thin film formed on a substrate having an insulating surface, said semiconductor thin film comprising a monodomain region having crystallinity that has been improved by illumination with laser light or strong light having equivalent energy thereto, the monodomain region being a collection of columnar or needle-like crystals extending generally parallel with the substrate.
According to another aspect of the invention, there is provided a semiconductor device which uses only the above monodomain region as an active layer. The monodomain region has a feature that it has substantially no grain boundaries.
According to a further aspect of the invention, there is provided a semiconductor device manufactured by a process comprising the steps of forming an amorphous silicon film on a substrate having an insulating surface by low-pressure thermal CVD; selectively forming a silicon oxide film on the amorphous silicon film; holding a metal element for accelerating crystallization of silicon adjacent to the amorphous silicon film: performing a heat treatment to convert at least part of the amorphous silicon film into a crystalline silicon film; removing the silicon oxide film; and illuminating the amorphous silicon film and/or the crystalline silicon film with laser light or strong light having equivalent energy thereto, to convert the crystalline silicon film into a monodomain region. The semiconductor device has an active layer that is constituted of only the monodomain region.
The present inventors define, as a monodomain region, a region which is obtained according to the invention by converting a lateral growth region and can substantially be regarded as a single crystal. The monodomain region has features that it contains substantially no grain boundaries and has almost no crystal defects such as dislocations and stacking faults.
xe2x80x9cSubstantially no grain boundariesxe2x80x9d means that grain boundaries are electrically inactive even if they exist. There have been found, as examples of such electrically inactive grain boundaries, a {111} twin crystal grain boundary, a {111} stacking fault, a {221} twin crystal grain boundary, a {221} twist twin grain boundary, etc. (R. Simokawa and Y. Hayashi, Japanese Journal of Applied Physics, Vol. 27, pp. 751-758, 1987).
The inventors consider that it is highly possible that grain boundaries in a monodomain region are electrically inactive grain boundaries as mentioned above. That is, they are considered an inactive region which does not obstruct carrier movement electrically, even though they appear to exist.
The monodomain region, which is the most important concept of the invention, is formed by the following process.
First, as shown in FIG. 1(A), crystal growth proceeds around a region 101 only in which a metal element has been introduced. The crystal growth proceeds generally parallel with a substrate, to form columnar or needle-like crystals.
The metal element for accelerating crystallization is one or a plurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. Ni (nickel) is used here as an example.
A lateral growth region 102 is formed in the above manner. For example, when a heat treatment is performed at 600xc2x0 C. for about 6 hours, the lateral growth length (X in FIG. 1(A)) reaches 100-200 xcexcm.
As shown in FIG. 1(A), the resulting lateral growth region 102 is divided into eight portions A-H, which appear as if each were a crystal grain. This is because defects such as slips occur at locations where the portions A-H collide with each other, to form crystal boundaries.
FIG. 1(B) is a schematic enlarged view showing a part of the portions A-H. As seen from FIG. 1(B), microscopically each portion of the lateral growth region is a collection of columnar or needle-like crystals. Since the columnar or needle-like crystals cluster together, each portion appears like a single crystal grain macroscopically.
Each of the columnar or needle-like crystals is a region which does not contain any grain boundaries and hence can be regarded as a single crystal, i.e., a monodomain region.
Since each crystal grows while removing impurity elements such as nickel from the inside, metal silicides are formed on the crystal surface. Thus, metal elements are segregated at grain boundaries 103 (see FIG. 1(B)).
Therefore, the state of FIG. 1(B) is a mere collection of monodomain regions. Although each portion of the lateral growth region has relatively superior crystallinity, it is not a monodomain region in itself.
To complete the invention, there is needed a step for improving the crystallinity of the lateral growth region 102. In this specification, this step is given a specific name xe2x80x9csingle-crystallization step.xe2x80x9d
Specifically, in the single-crystallization step of the invention, the crystalline silicon film obtained above is illuminated with laser light or strong light having equivalent energy.
It is desirable to use laser light emitted from an ultraviolet excimer laser. More specifically, a KrF excimer laser (wavelength: 248 nm), a XeCl excimer laser (wavelength: 308 nm), or the like may be used. Similar results can be obtained even by using strong light emitted from an ultraviolet lamp rather than laser light.
The surface of the crystalline silicon film illuminated with laser light is locally heated to a high temperature, and the silicon film is rendered in an instantaneous molten state. Actually, however, metal silicides segregated at the grain boundaries 103 between the columnar or needle-like crystals melt preferentially whereas the columnar or needle-like crystals do not melt easily.
That is, when the lateral growth region 102 shown in FIG. 1(B) is illuminated with laser light, the grain boundaries 103 preferentially melt, though instantaneously, and are then re-crystallized. In FIG. 1(C), dotted lines 104 indicate junction formed by temporary dissociation and subsequent recombination at the grain boundaries 103.
At this time, silicon lattices in the vicinity of the grain boundaries are rearranged and silicon atoms are thereby recombined in a well-matched manner. Therefore, as shown in FIG. 1(C), there remain substantially no grain boundaries in each of the portions A-H which was previously a collection of columnar or needle-like crystals as shown in FIG. 1(B).
Further, since crystal defects such as dislocations and stacking faults that previously existed in the columnar or needle-like crystals now disappear, the crystallinity of portions that were previously columnar or needle-like crystals is also improved remarkably.
At this time, the portions A-H expand in volume due to the rearrangement of silicon lattices. As a result, a phenomenon is observed that the silicon film protrudes at the grain boundaries where the portions A-H collide with each other (see FIG. 1(A)), i.e., at the peripheral portion of each monodomain region. The protrusion of the silicon film is one of the features associated with the above laser illumination step.
It is empirically known that the crystallinity in crystal grains is superior when the protrusion of a silicon film occurs at grain boundaries. However, the reason is not clear at present.
It has been found by SEM observations etc. that in case that the thickness of an amorphous silicon film is 500 xc3x85, for instance, the height of the protrusion of a silicon film is about 500 xc3x85.
The crystalline silicon film formed by the above process is greatly improved in crystallinity, and consists of monodomain regions whose crystallinity is equivalent to that of a single crystal.
One aspect of the invention is to form the active layer of a semiconductor device as typified by a thin-film transistor by using only a monodomain region as described above.
FIG. 4 shows active layers 404 arranged in matrix form on a substrate 401 having an insulating surface in manufacturing an active matrix liquid crystal display device.
Regions 402 indicated by broken lines are locations where regions for selective introduction of nickel existed. Reference numeral 403 indicate a location where a grain boundary formed by collision of lateral growth regions existed. The regions 402 and 403 are indicated by broken lines because they are unrecognizable after formation of the active layers 404.
As shown in FIG. 4, the active layers 404 of thin-film transistors are formed to assume a matrix form so as to avoid the nickel introduction regions and the grain boundary.
FIG. 4 is a local view, and the same things apply to all the active layers 404 formed on the substrate 401. That is, active layers of millions of thin-film transistors are formed by using only monodomain regions each containing no grain boundaries.